1. Field of the Invention
The present invention relates to a carbazole derivative having a heteroaromatic ring and to a light-emitting element, a light-emitting device, and an electronic device using the same.
2. Description of the Related Art
In recent years, intensive research and development have been made on a light-emitting element utilizing electroluminescence. A basic structure of the light-emitting element is a structure in which a layer containing a light-emitting substance is interposed between a pair of electrodes. Application of a voltage to the light-emitting element provides light emission from the light-emitting substance.
Since the light-emitting element is a self-emitting type element requiring no backlight and possesses advantages such as higher pixel visibility than a liquid crystal display, the light-emitting element has been considered to be suitable for not only the application to a lighting device but also to a flat panel display. In addition, it is also a great advantage that the light-emitting element can be manufactured as a thin and lightweight device. Further, extremely high response speed is also a feature of the light-emitting element.
Since the light-emitting element can be formed in a film shape, light emission from a flat surface with a large area can be readily obtained. Such a feature is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is significantly effective for use as a surface light source applicable to lighting device and the like.
The light-emitting element utilizing electroluminescence can be roughly classified according to whether the light-emitting substance is an organic compound or an inorganic compound. When an organic compound is used as the light-emitting substance, by application of a voltage to the light-emitting element, electrons and holes are injected from a pair of electrodes into a layer including the light-emitting organic compound, whereby a current flows. The carriers (electrons and holes) are recombined to allow the light-emitting organic compound to be excited, and light is emitted when the excited state relaxes to the ground state of the light-emitting organic compound.
Resulting from the above-mentioned mechanism, such a light-emitting element is called a current-excitation light-emitting element. Note that an excited state of an organic compound can be a singlet excited state and a triplet excited state. Luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. It is considered that the generation ratio of these two excited states in a light-emitting element is statistically S*:T*=1:3, that is, the generation ratio of the singlet excited state is 25%, while that of the triplet excited state is 75%.
A compound capable of emitting light from a singlet excited state (hereinafter, referred to as a fluorescent compound) generally does not emit light from its triplet excited state (phosphorescence) at room temperature and exhibits only luminescence (fluorescence) from the singlet excited state. Therefore, it is believed that the theoretical maximum of the internal quantum efficiency (ratio of generated photon to injected carriers) of a light-emitting element utilizing a fluorescent compound is 25% since the generation ratio of the singlet excited state in a light-emitting element is 25%.
On the other hand, the generation ratio of the triplet excited state in a light-emitting element reaches 75%, and some organic molecules existing in the singlet excited state are able to convert to the triplet excited state. Therefore, the use of a compound capable of emitting light from the triplet excited state (hereinafter, referred to as a phosphorescent compound) theoretically allows the internal quantum efficiency of a light-emitting element to be improved up to 75% to 100%, and luminous efficiency which is 3 times to 4 times as high as that using a fluorescent compound can be obtained. For these reasons, in order to achieve a light-emitting element with high efficiency, a light-emitting element using a phosphorescent compound has been intensively developed recently (Patent Document 1 and Non-Patent Document 1).
When a light-emitting layer of a light-emitting element is formed using the aforementioned phosphorescent compound, the phosphorescent compound is dispersed in a matrix formed of another material in most cases in order to suppress the concentration quenching of the phosphorescent compound and the triplet-triplet annihilation. In these cases, the material used to form the matrix is called a host material, and the material dispersed in the matrix like the phosphorescent material is called a guest material.
In the case where the phosphorescent compound is used as a guest material, the host material is required to have larger triplet excitation energy (a difference in energy between the ground state and the triplet excited state) than the phosphorescent compound. It is well know that CBP used as the host material in Non-Patent Document 1 has larger triplet excitation energy than the phosphorescent compound that emits light of green to red colors. Therefore, it is widely used as the host material for the phosphorescent compound.
However, although CBP has high triplet excitation energy, its insufficient ability to accept holes and electrons causes a problem in that driving voltage of the light-emitting element is increased. Therefore, a substance that has high triplet excitation energy and also can readily accept and transport both holes and electrons (i.e. a bipolar substance) is required as the host material for the phosphorescent compound.
In addition, since singlet excitation energy (an energy difference between the ground state and the singlet excited state) is larger than triplet excitation energy, a substance having high triplet excitation energy also possesses high singlet excitation energy. Therefore, the aforementioned substance, which has a bipolar property in addition to high triplet excitation energy, is effective in a light-emitting element using a fluorescent compound as the light-emitting substance.